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                                                                                                 SN54/74LS669
SYNCHRONOUS 4-BIT
UP/DOWN COUNTER
   The SN54 / 74LS669 is a synchronous 4-bit up/down counter. The LS669 is
a 4-bit binary counter. For high speed counting applications, this presettable
counter features an internal carry lookahead for cascading purposes. By
clocking all flip-flops simultaneously so the outputs change coincident with                          SYNCHRONOUS 4-BIT
each other (when instructed to do so by the count enable inputs and internal                          UP/DOWN COUNTER
gating) synchronous operation is provided. This helps to eliminate output
counting spikes, normally associated with asynchronous (ripple-clock) count-                          LOW POWER SCHOTTKY
ers. The four master-slave flip-flops are triggered on the rising (positive-going)
edge of the clock waveform by a buffered clock input.
   Circuitry of the load inputs allows loading with the carry-enable output of the
cascaded counters. Because loading is synchronous, disabling of the counter
by setting up a low level on the load input will cause the outputs to agree with
                                                                                                                      J SUFFIX
the data inputs after the next clock pulse.
                                                                                                                      CERAMIC
   Cascading counters for N-bit synchronous applications are provided by the                                         CASE 620-09
carry look-ahead circuitry, without additional gating. Two count-enable inputs                   16
and a carry output help accomplish this function. Count-enable inputs (P and                           1
T) must both be low to count. The level of the up-down input determines the
direction of the count. When the input level is low, the counter counts down,
and when the input is high, the count is up. Input T is fed forward to enable the                                     N SUFFIX
                                                                                                                      PLASTIC
carry output. The carry output will now produce a low level output pulse with a
                                                                                                                     CASE 648-08
duration  equal to the high portion of the QA output when counting up and                        16
when counting down  equal to the low portion of the QA output. This low level                          1
carry pulse may be utilized to enable successive cascaded stages. Regard-
less of the level of the clock input, transitions at the P or T inputs are allowed.
By diode-clamping all inputs, transmission line effects are minimized which                                            D SUFFIX
allows simplification of system design.                                                                                  SOIC
                                                                                                      16
   Any changes at control inputs (ENABLE P, ENABLE T, LOAD, UP/ DOWN)                                      1         CASE 751B-03
will have no effect on the operating mode until clocking occurs because of the
fully independant clock circuits. Whether enabled, disabled, loading or count-
ing, the function of the counter is dictated entirely by the conditions meeting
the stable setup and hold times.
                                                                                                      ORDERING INFORMATION

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